As a method of administering a drug transdermally by the use of a microneedle, there is a method, as disclosed in Patent Document 1, of kneading a drug into a material of a microneedle and then processing the resultant into a microneedle. On the other hand, Patent Document 2 describes a method of coating a drug on the surface of a microneedle by the medium of sugar.
In manufacturing a microneedle, since the material is often subjected to a process of change in shape by heating, thermally unstable drugs cannot be mixed into the material. In particular, for proteins, antibodies, vaccines, and the like, since they are thermally unstable, methods of manufacturing the microneedle are limited and difficult to be applied to mass production.
On the other hand, a method of applying a drug to a premade microneedle is generally a method, as disclosed in Patent Document 2, of immersing a microneedle in a solution in which a drug is dissolved. However, there is a difficulty to efficiently apply the drug to only small needles of the microneedle. Specifically, in order to uniformly immerse only the small needles of the microneedle in the drug liquid, a large drug liquid tank is required so as to level the surface of the drug liquid with respect to the small needles. When the drug liquid tank is large, the liquid surface is likely to be shaken due to vibration or wind. Therefore, the drug adheres to a base plate part of the small needles, resulting in uneven adhesion of the drug. The increase in size of the drug liquid tank requires a great amount of drug to be used, and therefore this is not realistic for expensive proteins. Conversely, when the drug liquid tank is small, as shown in FIG. 1, the liquid surface is convex due to surface tension; therefore, when the small needles are immersed therein, the immersed area of the small needles is different between the center part and the peripheral part of the tank. Moreover, when the drug liquid is an aqueous solution, the effect of surface tension is especially high. Thus, it is observed that, even though the drug liquid can be applied to the small needles in the center part, it cannot be sufficiently applied to the small needles in the peripheral part.
While a wide variety of methods of manufacturing a microneedle have been studied and reported, a method of applying a drug liquid to a premade microneedle has been rarely reported. Furthermore, a method of application which can be industrially mass-produced in a realistic way has not been reported, and various methods are still now under research and study.
Patent Document 1: WO2006/080508
Patent Document 2: JP 2004-504120A